1. Field of the Invention
This invention relates to a device for driving to move a focusing lens forward or backword depending upon the focus condition, i.e., whether it is too close, in-focus or too far, and, in particular, it relates to a lens driving device of an auto-focus system for use in a still camera, cinematographic camera (8 mm, 16 mm, etc.), video camera and the like to move the focusing lens into the in-focus condition.
2. Description of the Prior Art
In general, in an auto-focus system, a distance to an object is measured, and the thus obtained distance information is processed to determine whether it is too close, in-focus or too far, on the basis of which the current focus condition is indicated in an indicator and the position of the focusing lens is controlled (forward movement, no movement or backward movement). And, this series of steps is repetitively carried out.
FIG. 1 schematically shows a distance measuring system when applied to a single reflex camera. As shown, a quick return mirror or main mirror 1 is provided with a sub-mirror 2 which reflects the light coming through the main mirror 1 toward the bottom portion of a mirror box, where a beam splitter 3 is located, as shown in FIG. 2. Below the beam splitter 3 is defined a focusing reference surface F' which is located optically equidistantly as a film surface F with respect to the main mirror 2. Provided as attached to the beam splitter 3 is a pair of solid-state image sensors S.sub.1 and S.sub.2, each comprised of a charge coupled device (CCD) having a train of sensing elements. The CCDs S.sub.1 and S.sub.2 are optically located on both sides of and equidistantly from the focusing reference surface F'. These CCDs S.sub.1 and S.sub.2 scan the illumination distribution of an object of interest and the sensed values between the corresponding sensing elements are then compared to produce a contrast evaluation function.
By changing the position of the lens with respect to the film surface F, or the focusing condition, a contrast curve C.sub.1 is obtained from the image sensor S.sub.1, which is located closer to the mirror 2 and thus the focusing lens with respect to the focusing reference surface F' or film surface F, and the thus obtained contrast curve is graphically shown in FIG. 3(a). Similarly, a contrast curve C.sub.2 may be obtained from the other image sensor S.sub.2, and this curve is also shown in FIG. 3(a). Since the image sensors S.sub.1 and S.sub.2 are located in front of (too close side) and aft of (too far side) the focusing reference surface F', respectively, the curves C.sub.1 and C.sub.2 are out of phase, as shown in FIG. 3(a). Since the image sensors S.sub.1 and S.sub.2 are equidistantly located from the focusing reference surface F', if the focusing lens, initially in the too close region, is moved toward the too far region, the contrast value C.sub.1 obtained from the image sensor S.sub.1 gradually increases to reach its maximum and then starts to go down. On the other hand, the contrast value C.sub.2 obtained from the image sensor S.sub.2 also gradually increases and it reaches its maximum after the contrast curve C.sub.1 having already passed its own maximum. Thereafter, the contrast curve C.sub.2 also goes down.
Under the circumstances, the merging point between the two curves C.sub.1 and C.sub.2, which is a midpoint between the respective maximum points, indicates that the object of interest is sharply focused on the film surface F. On the other hand, the condition of C.sub.1 greater than C.sub.2 indicates the too close condition and the condition of C.sub.1 smaller than C.sub.2 indicates the too far condition. Thus, in order to determine such a focusing condition, a differential curve C.sub.3 is obtained by taking a difference between the contrast curves C.sub.1 and C.sub.2, as shown in FIG. 3(c), and by comparing the thus obtained differential curve C.sub.3 with predetermined threshold levels T.sub.1 (positive) and T.sub.2 (negative), it is determined to be in focus if the value of C.sub.3 is in the range between T.sub.1 and T.sub.2, or nearly equal to zero; on the other hand, it is determined to be too close, if C.sub.3 is above T.sub.1, and it is determined to be too far, if C.sub.3 is below T.sub.2. The regions indicated as "BEYOND RANGE" in FIG. 3(a) cannot be used because the contrast level is extremely low in these regions. It is true that the differential curve C.sub.3 approaches zero also in these regions; however, this condition is secluded from the in-focus condition by watching the behavior of the two contrast curves C.sub.1 and C.sub.2.
FIG. 3(b) schematically shows an example of the indicator for indicating the current focusing condition, whether it is too close, in-focus or too far, from left to right, and such an indicator may be provided in a viewfinder of a camera. In FIGS. 2 and 3, the distance between the film surface F and the detecting surface of each of the image sensors S.sub.1 and S.sub.2 is denoted by 1. The focusing condition thus determined is then indicated in a separate indicator or the viewfinder, and the operator may move the focusing lens either toward or away from the film surface depending upon the current focusing condition to bring the lens into the in-focus position. Alternatively, in the case where an automatic lens moving mechanism is provided, such a detected signal may be used to bring the lens into the required in-focus position automatically.
FIG. 4 shows in block diagram a typical prior art detection/control mechanism in an auto-focus system, and it includes a light receiving section 4 comprised of such elements as image sensors S.sub.1 and S.sub.2, an arithmetic processing circuit 5 for producing the contrast curves C.sub.1 and C.sub.2 on the basis of the distance information detected by the light receiving section 4 to compare the difference between the two curves C.sub.1 and C.sub.2 with predetermined threshold levels T.sub.1 and T.sub.2 to determine the current focusing condition to be too close, in focus or too far and an indication/control section 6 for indicating the current focusing condition or controlling the movement of the focusing lens in response to a signal supplied from the arithmetic processing circuit 5. The operation of the mechanism shown in FIG. 4 includes a series of a light receiving (distance measuring) step, an arithmetic processing (determination of the current focusing condition step and an indicating and controlling step, and such a series of steps is carried out in repetition.
The focusing accuracy of the structure shown in FIG. 4 may be enhanced by setting the threshold levels substantially closer to zero. In such a case, however, the focusing condition indicator rapidly changes its indicated state, in focus or out of focus, for example, when the camera is held by the operator's hands or the object of interest is in motion. If this happens, it is very difficult to properly judge the current focusing condition. Moreover, the focusing lens is driven to move quickly forward and backward in a reciprocating manner, and a motor for driving to move the focusing lens is set in vibration, thereby causing unstability in operation. And, from a practical viewpoint, it is not always required to obtain the highest accuracy, though it is better to have a possible highest accuracy at all times. On the other hand, the threshold levels T.sub.1 and T.sub.2 may be set larger in absolute value so as to allow to judge the current focusing condition easily, but, in this case, the focusing accuracy becomes lowered and unsatisfactory.